Presently available high-reflectivity mirrors have serious commercial and military limitations. Such mirrors are first surface mirrors and consist primarily of a thin film of reflective metal deposited on the surface of a carefully polished glass section, generally referred to as a substrate. The mirror surface follows the glass surface whether flat, concave, convex, spherical, or aspherical.
Freshly deposited silver has the best reflectance of the 0.4 to 1.1 micron spectrum, but it deteriorates with time. Freshly deposited copper and gold have a poor reflectance over much of the visible spectrum, but are excellent reflectors over the near infrared range. Aluminum has considerably less reflectance than copper, gold, or silver over the near infrared, but has good reflectance over the visible spectrum. Aluminum has the inherent quality of quickly oxidizing its surface with a coating of aluminum oxide which protects the reflectance of the metallic aluminum without appreciably reducing its reflectance. This characteristic of aluminum to maintain its surface with time has led to the use of aluminum as the reflector for commercial applications.
Aluminum mirror surfaces, however, have a serious commercial drawback: aluminum is a soft metal, and even when coated with a hard coating, the cleaning of the surface causes deterioration after repeated cleanings.
Copper, by contrast, is a comparatively hard metal and when coated may be readily cleaned without deterioration after repeated cleanings. Pure copper is harder and more durable than pure aluminum; therefore, the coated copper surface is harder and more durable than the coated aluminum surface. However, copper's pure reflectance over much of the visible range has heretofore limited its use to special applications.